In recent years, reinforcing fiber materials such as carbon fiber, glass fiber, and aramid fiber have been made into composites with various types of matrix resins, and the fiber-reinforced plastics thus obtained have been widely used in various types of fields and applications. In the aerospace field where a high level of mechanical properties, heat resistance, etc. are required, and in the general industrial field, a thermosetting resin such as an unsaturated polyester resin, an epoxy resin, or a polyimide resin has conventionally been used as a matrix resin. However, in the aerospace field in particular, there has been a need to improve these matrix resins due to drawbacks such as brittleness and poor impact resistance. Furthermore, in the case of a thermosetting resin, when this is made into a prepreg, there is a problem with storage management of the prepreg due to the short shelf life of the resin, and there are also the problems that the ability to follow product shape is poor, the molding time is long thus giving low productivity, etc. On the other hand, in the case of a thermoplastic resin prepreg, impact resistance is excellent when it is in a composite material, prepreg storage management is easy, the molding time is short, and there is a possibility of the molding cost being reduced.
With regard to the fiber-reinforced thermoplastic resin prepreg employing a thermoplastic resin as a matrix, there are the following types from the viewpoint of the morphology of the reinforcing fiber and its orientation. There is (1) a prepreg formed from a thermoplastic resin and a reinforcing continuous fiber that is a unidirectionally aligned fiber sheet or a woven/knitted fabric, formed using a continuous fiber. Such a prepreg has the advantage that the fiber volume fraction can be increased and has excellent performance in terms of modulus of elasticity and strength in the fiber axis direction. However, when a single prepreg is considered, it is in-plane anisotropic, and when a product is molded, in order to ensure isotropy in its mechanical properties it is necessary to layer a plurality of prepregs in the fiber axis direction, directions perpendicular thereto, and directions oblique thereto. This inevitably causes an increase in the cost, and results in the problem that the products becomes thick. Furthermore, there is (2) a prepreg (chopped strand prepreg) employing chopped strands in which discontinuous fibers are used as the reinforcing fiber. This is a small-piece prepreg formed by cutting unidirectionally aligned strands (fiber bundles) employing a thermoplastic resin as a matrix into a fiber length of, for example, on the order of 25 mm to 50 mm. Such a prepreg has good flowability during molding, for example, flowability during stamping, and is suitable for producing moldings with various complicated shapes. However, since it is essential to employ a mechanism for fluidizing the reinforcing fiber with the resin during molding, there is the problem that the volume fraction of the reinforcing fiber cannot be made high. Furthermore, there is the problem that, since the volume fraction of the reinforcing fiber is low and the reinforcing fiber is short, properties such as modulus of elasticity and strength are poor compared with one employing a continuous fiber.
JP-A-9-155862 (Patent Publication 1) (JP-A denotes a Japanese unexamined patent application publication) states that, in order to provide a fiber-reinforced thermoplastic resin sheet having a high weight fraction of a reinforcing fiber and good dispersion properties, for which the strength and the modulus of elasticity are pseudoisotropic within the plane and the flowability of the reinforcing fiber during post-processing is good, it is necessary to satisfy the following three requirements, that is, (1) the weight fraction of the reinforcing fiber is 50% to 85% and the weight fraction of the thermoplastic resin is 15% to 50%, (2) the average fiber length of the reinforcing fiber is 5 mm to 50 mm, and (3) the reinforcing fibers are dispersed non-directionally. However, Patent Publication 1 employs glass fiber in particular as the reinforcing fiber and has the object of increasing the weight fraction of the glass fiber, and does not propose one having excellent mechanical properties even with a low volume fraction of the reinforcing fiber.
(Patent Publication 1) JP-A-9-155862